Battery Time-of-Use Arbitrage: The Real Math

Charge the battery when electricity is cheap, discharge it when electricity is dear, and pocket the spread — that is where home battery time of use arbitrage savings come from, and it is the most oversold idea in residential energy storage. The pitch sounds like free money. The reality, worked out with 2026 rate schedules, is more interesting and more honest: arbitrage produces real savings — typically hundreds of dollars a year for a standard home battery, not thousands — and whether it is worth chasing depends on your utility's rate spread, your battery's efficiency, and what the daily cycling costs in battery life.

This guide does the math out loud with a worked 2026 example, accounts for round-trip efficiency losses and cycle-life trade-offs, and shows exactly when arbitrage pays and when it is a distraction from the battery's better jobs. No guaranteed paybacks, no free money — just arithmetic. Costs are 2026 US market ranges; get itemized local quotes.

The idea in one paragraph

Time-of-use (TOU) rates charge different prices for electricity at different hours: cheap overnight and midday, expensive during the evening peak — typically 4–9 p.m. in California, with similar windows spreading across utilities nationwide. A battery arbitrages that spread by charging during the cheap hours (from the grid overnight or from solar midday) and discharging during the expensive hours to run the house, so each stored kilowatt-hour displaces a kilowatt-hour you would have bought at peak price. The profit per cycle is the peak price minus the off-peak price, minus the energy lost to inefficiency, times the kilowatt-hours shifted. Every term in that sentence matters, and the rest of this guide quantifies each one.

Home battery time of use arbitrage savings: a worked 2026 example

Let us build the example with realistic 2026 California numbers, since California's TOU spreads are among the widest in the country — and therefore the best case for arbitrage. Take PG&E's E-TOU-C residential plan in summer: peak pricing runs 4–9 p.m. daily at roughly 40–52 cents per kWh depending on baseline tier, while off-peak sits around 32 cents. Call the summer spread 16 cents per kWh — the difference between buying at 48 and avoiding buying at 32 is the territory we are in. Winter spreads are narrower, roughly 6–8 cents. (Your utility's tariff sheet is the authority here; rates move, and this example is arithmetic illustration, not a quote.)

Now the battery: a 13.5 kWh all-in-one unit, the common residential class. Assume a full daily cycle — charge 13.5 kWh overnight at 32 cents, discharge during the 4–9 p.m. peak. But the battery is not a perfect bucket: round-trip efficiency of about 90 percent means 13.5 kWh in becomes about 12.15 kWh out. The daily economics:

Cost to charge: 13.5 kWh × $0.32 = $4.32
Value of discharge: 12.15 kWh × $0.48 = $5.83
Net per summer day: $5.83 − $4.32 = $1.51

Over a 122-day summer season, that is about $184. Run the same math for winter with a 7-cent spread: net per day lands around $0.30–$0.50, or roughly $75–$120 over the winter season. Total: on the order of $260–$300 per year for a 13.5 kWh battery cycling fully every day on this tariff. In a higher-spread territory — some SCE summer peak windows reach the mid-40-cent range against low-30s off-peak — the same battery might clear $400–$600 a year. In a flat-rate or narrow-spread territory, it might clear $100.

There is the honest headline: a standard home battery earns roughly $250–$600 a year from pure rate arbitrage in favorable 2026 markets. Real money — but set against a $12,000–$20,000 installed battery, arbitrage alone is a decades-long payback. Nobody should buy a battery for arbitrage. They should buy it for backup or solar self-consumption, and treat arbitrage as a yield kicker. Anyone telling you otherwise is selling something.

The round-trip tax: efficiency losses on every cycle

The worked example already includes it, but the efficiency term deserves its own spotlight because it is the silent partner in every arbitrage calculation. Round-trip efficiency — the fraction of energy you get back from energy you put in — runs about 85–90 percent for modern lithium home batteries (AC-to-AC; DC-coupled solar charging can be a point or two better). That missing 10–15 percent is lost as heat in the inverter, the battery chemistry, and the wiring, on every cycle, forever.

Efficiency does two things to the math. First, it shrinks the effective spread: you are not arbitraging peak-minus-off-peak, you are arbitraging (peak × efficiency) − off-peak. With a 16-cent nominal spread and 90 percent efficiency, the effective spread is about 11 cents per stored kWh. Second, it sets a floor on the spread you need: if the peak-to-off-peak spread is smaller than the efficiency loss expressed in cents, arbitrage loses money — you would have been better off just buying peak power. At 90 percent efficiency and 32-cent off-peak power, the peak price needs to exceed about 35.5 cents before the first dollar of profit appears. Narrow-spread utilities never clear that bar, which is why arbitrage is a California-and-friends game, not a national one.

The capacity constraint: you can only shift what you store

The second binding constraint is embarrassingly simple: arbitrage revenue scales with usable capacity, and residential batteries are small relative to evening consumption. A home pulling 2 kW through the five-hour peak window consumes 10 kWh — most of a 13.5 kWh battery. A home with central air conditioning pulling 4 kW through the same window consumes 20 kWh, exceeding the battery entirely; the excess is bought at peak rates no matter what the battery does.

This creates a sizing tension nobody advertises: the battery that is perfectly sized for overnight backup is often undersized for full peak-shaving, and vice versa. Practical arbitrage means covering as much of the peak window as the battery holds, not the whole window — partial peak-shaving still earns the spread on every shifted kWh. Smart energy management helps at the margin: pre-cooling the house before 4 p.m. so the AC idles through the peak, shifting the dishwasher and EV charging to off-peak hours, and letting the battery handle the residual. The battery arbitrages what behavior does not eliminate.

There is also the reserve conflict. Every kWh reserved for backup — the 20–30 percent many owners hold against outages — is a kWh not arbitraging. A 13.5 kWh battery with a 25 percent backup reserve arbitrages about 10 kWh, cutting the worked example's revenue by roughly a quarter. During storm season, prudent owners raise the reserve and accept lower arbitrage yield; the battery's jobs have a pecking order, and backup outranks bill-shaving. Size the reserve for your outage risk first, then arbitrage with the remainder.

Cycle life: every arbitrage dollar spends battery life

Here is the trade-off the sales brochures skip: arbitrage means cycling the battery daily — 365 full or partial cycles a year — and cycles are the currency batteries spend. Modern LFP home batteries are typically warranted for 15 years or around 6,000 cycles (to 60 percent capacity, on the leading warranty in the class); some warranties are throughput-limited instead, such as 60 MWh total. A daily full arbitrage cycle consumes about 365 cycles a year — roughly 6 percent of a 6,000-cycle warranty life annually, or about 5 MWh of a 60 MWh throughput allowance.

Run the economics on that: $300 a year in arbitrage revenue against ~6 percent of the battery's warranted life per year implies the battery's lifetime arbitrage value is on the order of $4,000–$5,000 — real, but a fraction of the battery's cost, and it assumes the battery would otherwise sit idle, which it would not (backup readiness and self-consumption have their own value). The deeper point: daily deep cycling for arbitrage accelerates the capacity fade that eventually retires the battery. Partial cycling — arbitraging the middle 60 percent of the battery rather than 10-to-100 percent daily — earns most of the revenue with meaningfully less wear, and it is what most energy-management defaults actually do.

The honest framing: arbitrage does not destroy batteries — they are designed to cycle — but it converts a slowly-depreciating backup asset into a working asset, and the wage is modest. If your utility's spread is wide and your battery would otherwise sit at 100 percent doing nothing all summer, arbitrage is found money. If you are in a narrow-spread territory, the cycles are worth more saved for backup longevity than spent chasing dimes.

When the math works — and when it doesn't

Scenario	Typical spread	Annual arbitrage (13.5 kWh battery)	Verdict
California IOU, strong TOU (PG&E/SCE summer)	14–20¢/kWh	$300–$600	Worth doing — meaningful yield kicker
Moderate TOU (many municipal utilities)	6–10¢/kWh	$100–$250	Marginal — do it if automated, don't chase it
Flat-rate territory	~0¢/kWh	$0	No arbitrage case at all
Narrow spread + high reserve need (storm country)	Any	Reduced by reserve	Backup first; arbitrage with leftovers
Solar + NEM 3.0 / net billing	Implicit (avoided peak purchase)	Often beats pure arbitrage	Self-consumption is usually the better game

The last row is the one most owners should actually focus on. Under net-billing regimes where exported solar earns little, the battery's highest-value daily job is usually self-consumption — storing midday solar and discharging it through the evening peak — which is arbitrage's close cousin but with free charging energy instead of 32-cent grid energy. The worked example's $4.32 daily charging cost drops to zero when the sun provides it, roughly doubling the net. If you have solar, optimize for self-consumption first and treat grid-charged arbitrage as the shoulder-season supplement.

Stacking value: the battery's real paycheck

Step back and the picture clarifies: no single battery revenue stream justifies the purchase alone, but the stack can be compelling. Backup power is the insurance value — hard to price, obvious the first time the grid fails for two days. Solar self-consumption under net billing is usually the largest financial return, often several hundred dollars a year in avoided peak purchases. Rate arbitrage adds its $250–$600 in wide-spread territories. And in some markets, virtual power plant programs pay batteries to discharge during grid emergencies — worth investigating locally, with the usual caution to check current availability rather than assume.

Evaluate the battery on the stack, sized to your home's actual loads, installed by a licensed professional with permits and utility interconnection handled. The arbitrage math in this guide is one line item in that evaluation — now you can compute it yourself, which is more than most sales presentations will do for you.

Next steps

Pull your utility's current TOU tariff sheet — not last year's, not a summary — and compute your peak-to-off-peak spread for summer and winter separately. Multiply the effective spread ((peak × 0.90) − off-peak) by the kWh you would actually cycle daily after reserving backup capacity, times 365. That number is your personal arbitrage ceiling. Then get itemized battery quotes and evaluate the system on the full value stack: backup, self-consumption, arbitrage, and any local programs.

And keep the hierarchy straight: buy the battery for resilience and solar value, let arbitrage sweeten the return, and never let a daily cycling schedule compromise the backup reserve your household actually needs. The spread is real, the math is honest, and now it is yours.

Frequently asked questions